The pressure swing adsorption ("PSA") process has been widely used for gas separation and purification. More specifically, the PSA process is used for oxygen production from air. The basic process is carried out in fixed bed adsorbers and involves a sequence of cyclic operating steps of high pressure adsorption of nitrogen for oxygen production followed by low pressure desorption of nitrogen for adsorbent regeneration. The process has many different configurations and operating procedures, depending on the desired trade-off between product recovery and purity and system complexity.
The present process has also been developed in connection with removing nitrogen ("N.sub.2 ") from air to yield significantly purified oxygen ("O.sub.2 "). It will be described hereinbelow in connection with that particular feedstock. However it is contemplated that the invention may find application with respect to treating other gas mixtures as well.
The typical PSA process employs a set of adsorbers arranged in parallel flow relationship with each adsorber proceeding sequentially through a multi-step adsorption/desorption cycle. The adsorption step of an adsorber is generally synchronized with the desorption step of other adsorbers so that O.sub.2 can be produced continuously. A typical PSA system used for the separation of the N.sub.2 and O.sub. 2 components of air involves 3 to 4 adsorber columns ("adsorbers") arranged in parallel flow relationship. The adsorbers are each packed with a bed of adsorbent particles. This system is illustrated in U.S. Pat. Nos. 3,430,418 (Wagner), 3,564,816 and 3,636,679 (Batta) and 3,973,931 (Collins). About one to two dozen timer controlled switching valves are required in each of these systems to implement and control the cyclic sorption process. Each adsorber is operated to go through the following basic steps in sequence in a manner such that the system is continuously accepting feed air and producing O.sub.2 -enriched product gas:
(1) Introduction of air into the adsorber, pressurization thereof and adsorption of N.sub.2 from the feed air to produce O.sub.2 -enriched product gas exiting from the adsorber outlet; PA1 (2) Co-current de-pressurization ("co-current" means in the same direction as the feed air flow during adsorption) to recover O.sub.2 enriched product gas still remaining in the adsorber at the end of the adsorption step, for use in pressurizing or purging other adsorbers. This usually involves multi-step de-pressurization of the adsorber and transfer of the depressurized gas to several other adsorbers through their outlet ends for the purpose of purging or re-pressurization prior to adsorption; PA1 (3) Counter-current venting to remove adsorbed N.sub.2 from the adsorbent by blowing down the adsorber to ambient pressure through its feed air inlet; PA1 (4) Counter-current purging of the blown down adsorber to further remove the adsorbed N.sub.2 using the purified product gas obtained from another adsorber undergoing co-current de-pressurization; and PA1 (5) Counter-current re-pressurization of the purged adsorber using part of the product gas or the gas obtained from another adsorber undergoing co-current de-pressurization. PA1 (a) During the adsorption step, there develops an oxygen concentration gradient along the length of the adsorber column, increasing in the direction of the feed air flow; the oxygen concentrations at the feed inlet and product outlet of the column are equal to that of the feed air and product gas, respectively. The concentration gradient of nitrogen is just opposite to that of oxygen; it increases in the opposite direction, toward the feed inlet. Therefore, during co-current de-pressurization, which follows the adsorption step, the nitrogen content in the gas exiting from the adsorber outlet always gradually increases with time. Hence, the more the gas is extracted from the adsorber by co-current de-pressurization, the higher is its nitrogen content; PA1 (b) The gas obtained from the co-current de-pressurization of an adsorber is usually transferred to other adsorber columns (referred to as `recipient adsorbers`) through their product outlet ends for purging or re-pressurization. Since the nitrogen content in the said gas generally increases with time, this always creates an undesirable oxygen concentration profile in the recipient adsorber being purged or re-pressurized--the oxygen concentration decreases toward the product outlet end. In other words, the oxygen concentration gradient created by purging or re-pressurization of the recipient adsorber using the co-currently depressurized gas from another adsorber is always opposite to that developed during the adsorption step as illustrated in FIG. 4; PA1 (c) The nitrogen contamination of the recipient adsorber outlet due to the oxygen gradient reversal, created by the purging and/or re-pressurization using the co-currently depressurized gas, will adversely affect performance of the subsequent adsorption step in the recipient adsorber, making it difficult to achieve the desired product oxygen purity. This can only be avoided by terminating the co-current de-pressurization step earlier, and stopping at a higher pressure. But this imposes a serious limit on the recovery and utilization of the already partially oxygen-enriched gas remaining in the adsorber at the end of the adsorption step. PA1 (1) feeding a charge of feed gas, comprising a mixture of product and contaminant components, into the first adsorber, with the adsorber's outlet valve closed, and temporarily retaining the charge in the adsorber and pressurizing it to a predetermined pressure, so that part of the contaminant component is adsorbed by the adsorber's adsorbent to form a product-enriched gas having an increasing product component concentration gradient and a decreasing contaminant component concentration gradient extending from the adsorber's inlet to its outlet; PA1 (2) co-currently depressurizing the first adsorber by venting product-enriched gas through the second adsorber and product reservoir vessel, preferably through the second adsorber and product reservoir vessel one at a time; PA1 (3) counter-currently venting the adsorbers, preferably one at a time, through the inlet of the first adsorber, to purge them; PA1 (4) counter-currently and partly re-pressurizing the adsorbers with product-enriched gas from the reservoir vessel; and PA1 (5) repeatedly repeating the cycle of steps (1) through (4) to recover product-enriched gas from the reservoir vessel. PA1 (a) By operating the process columns serially, reversal of the component concentration gradients is avoided. More particularly, as a result of using only serial gas transfer between the adsorbers, the following occur: during feed gas re-pressurization and co-current de-pressurization both the contaminant and product component concentration gradients in the adsorbers develop in the co-current direction and create an increasing product component concentration gradient and a decreasing contaminant component concentration gradient extending from adsorber inlet to outlet. During counter-current venting and partial re-pressurization, these gradients retract in the opposite direction but the patterns of decreasing contaminant component concentration and increasing product component concentration gradients are preserved from adsorber inlet to outlet; PA1 (b) By isolating the first adsorber from the second adsorber during feed gas pressurization, the first adsorber may be fully pressurized by the feed air to any pressure level for any length of time without the danger of contaminant breakthrough to the product vessel. Thus, the process operating cycle time can be flexible, and the adsorption capacity of the entire first adsorber can be fully utilized for gas separation. In comparison, for the single-column PSA (e.g. Jones, PA1 (c) The presence of the second adsorber makes it possible to co-currently depressurize the first adsorber to a very low pressure level prior to the counter-current venting step. This is a direct consequence of the operating procedure of feeding the air to only a single (first) adsorber and venting the nitrogen-enriched residual air from both first and second adsorbers. It can be shown by material balance of air feed and residue vent that in the absence of the second adsorber (as in the single-column PSA process), the maximum: possible co-current de-pressurization will only reduce the pressure in the first adsorber by at most 50%, regardless of the size of the product vessel with acts as the receiver for the depressurized gas. In contrast, the present process with the second adsorber in place can easily reduce the pressure in the first adsorber by about 75% through co-current de-pressurization. Such effective co-current de-pressurization enables an improvement in process efficiency by increased recovery and utilization of the product-enriched gas in the first adsorber after pressurization; PA1 (d) By co-currently depressurizing the first adsorber through the second adsorber and product reservoir vessel sequentially one at a time, the second adsorber can have the beneficial effect of being pressurized to an intermediate pressure level prior to depressurizing to the product vessel. The increased adsorbing capacity of the adsorbent in the second adsorber due to the said pressurization can reduce the level of contaminant in the product gas issuing therefrom. PA1 (e) By preferably venting the first and second adsorbers sequentially one at a time, the first adsorber will reach ambient pressure before venting of the second adsorber begins--this facilitates maximum expansion of the gas from the second adsorber, which therefore provides a large volume of purging gas for the first adsorber. PA1 (f) The present process can be conducted using fewer switching valves and interconnecting lines than the conventional multibed PSA processes (see comparative Example V below).
So, in general, the prior art technology involves pressurization and adsorption, co-current de-pressurization, counter-current venting, counter-current purging, and counter-current re-pressurization.
Underlying the prior art multi-bed PSA technology, one of the common themes of hardware design and operating method is to maximize the utilization of the O.sub.2 -enriched (or purified) product gas still trapped in the mass transfer zone of the adsorber following the adsorption step. The inter-adsorber pressure equalization method is generally used to facilitate the recovery and utilization of this gas. In order to increase the effectiveness of this recovery/utilization process, it is necessary to increase the complexity of the system using a large number of switching valves to carry out multi-step pressure equalization for several pairs of adsorbers at progressively lower pressure levels.
The following shortcomings can be identified in the existing multi-bed PSA processes:.
Most existing PSA processes suffer from the drawback of oxygen concentration gradient reversal in their purging and/or re-pressurizing step. A few of the PSA systems have managed to avoid this shortcoming in the purging step by using an empty or adsorbent-filled vessel as a temporary storage vessel for the gas obtained from the adsorber undergoing co-current de-pressurization. The gas is then withdrawn from the said vessel in reverse direction and used for purging the same adsorber following its counter-current venting step (e.g. see U.S. Pat. Nos. 3,142,547 (Marsh et al), and 3,788,036 (Lee et al)). But those systems still suffer from the same said drawback in their re-pressurization step which involves gas transfer through the product outlet ends of two adsorbers. In the PSA system described in U.S. Pat. 4,715,867 (Vo), the problem of oxygen concentration gradient reversal is partially mitigated by feeding the co-currently depressurized gas from one adsorption zone to the mid-point of the other adsorption zone (rather than through the product outlet end), the said adsorption zone being defined as a pair of two serially connected adsorber columns. Half of the adsorption zone, however, is still susceptible to oxygen concentration gradient reversal.
In the simple single-column PSA processes (e.g. U.S. Pat. Nos. 4,194,892 (Jones et al), and 4,892,566 (Bansal et al)), which do not involve any gas transfer between adsorbers, there is no possibility of oxygen concentration reversal. But this advantage is more than offset by their inability to carry out the important co-current de-pressurization process. As a result, the performance of the single-column process is considerably inferior to the multi-bed process.
With this background in mind, it is the objective of the present invention to provide a process designed to eliminate the undesirable contaminant component concentration gradient reversal and to carry out more effective co-current de-pressurization.